Cation and/or anion substitution

Orimo’s group, motivated by the studies on the thermal stability of borohydrides [Reference Stephenson, Rice and Stockmayer107], performed systematic investigation of the LiBH₄–LiX systems, where a fraction of [BH₄]⁻ was replaced by halide anions (X = Br⁻, Cl⁻, and I⁻) [Reference Matsuo and Orimo11, Reference Unemoto, Matsuo and Orimo12, Reference Maekawa, Matsuo, Takamura, Ando, Noda, Karahashi and Orimo14, Reference Miyazaki, Karahashi, Kumatani, Noda, Ando, Takamura, Matsuo, Orimo and Maekawa15]. The (1 − x)Li(BH₄) + xLiI solid solution with a hexagonal crystal structure was successfully obtained by a mechanochemical synthesis (Table I). The replacement of [BH₄]⁻ for I⁻ stabilized the formation of the HT phase at lower temperatures, which decreased monotonically with an increasing concentration of the I⁻ ions. As demonstrated by powder X-ray diffraction (PXD), for x > 13%, the HT phase was observed at RT. The sample with 40% of the LiI content showed σ as high as 0.1 × 10⁻³ S/m and 10 × 10⁻³ S/m at 303 K and 413 K, respectively [Reference Matsuo and Orimo11, Reference Unemoto, Matsuo and Orimo12, Reference Sveinbjornsson, Myrdal, Blanchard, Bentzen, Hirata, Mogensen, Norby, Orimo and Vegge13, Reference Maekawa, Matsuo, Takamura, Ando, Noda, Karahashi and Orimo14, Reference Miyazaki, Karahashi, Kumatani, Noda, Ando, Takamura, Matsuo, Orimo and Maekawa15, Reference Oguchi, Matsuo, Hummelshoj, Vegge, Norskov, Sato, Miura, Takamura, Maekawa and Orimo108]. Later, it was confirmed that a higher fraction of I⁻ anions reduced the activation energy and subsequently increased the conductivity of Li⁺ [Reference Skripov, Soloninin, Rude, Jensen and Filinchuk109, Reference Epp and Wilkening110]. On the other hand, the higher mobility of Li⁺ in Li(BH₄)_1−nI_n, compared to LT-LiBH₄, was assigned to the I⁻-induced lattice anharmonicity and the increased distance between neighboring [BH₄]⁻ units [Reference Buchter, Lodziana, Mauron, Remhof, Friedrichs, Borgschulte, Zuttel, Sheptyakov, Strassle and Ramirez-Cuesta105, Reference Filinchuk, Chernyshov and Cerny111]. The formation of solid solutions with enhanced Li-ion conductivity was also observed in the LiBH₄–LiBr and LiBH₄–LiCl systems [Reference Maekawa, Matsuo, Takamura, Ando, Noda, Karahashi and Orimo14, Reference Cascallana-Matias, Keen, Cussen and Gregory16, Reference Matsuo, Takamura, Maekawa, Li and Orimo17]. Additionally, in the case of Li(BH₄)_1−nBr_n, the importance of applied synthesis methods (mechanochemical powder processing alone over combination with thermal annealing) on the ionic transport properties was emphasized [Reference Cascallana-Matias, Keen, Cussen and Gregory16].

The successful anionic substitution that stabilized the HT-LiBH₄ down to RT initiated a broad research activity on LiBH₄-related compositions, obtained by either a cation and/or an intermolecular anion substitution.

In 2009, Matsuo et al. reported on the high ionic conductivity of Li⁺ in trigonal Li₂(BH₄)(NH₂) and cubic Li₄(BH₄)(NH₂)₃ borohydride amides, prepared by mechanochemistry. The simultaneous presence of [BH₄]⁻ and [NH₂]⁻ complexes resulted in multiple crystallographic sites for the Li⁺ cations and increased their mobility to 2 × 10⁻⁴ S/cm at RT in both compounds [Reference Matsuo, Remhof, Martelli, Caputo, Ernst, Miura, Sato, Oguchi, Maekawa, Takamura, Borgschulte, Zuttel and Orimo20]. In addition, the study showed that the Li₂(BH₄)(NH₂) activation energy, which is one of the measures of the barriers for lithium-ion migration, decreased from 0.66 to 0.24 eV at 368 K as a result of the compound melting. The σ value in the resulting Li₂(BH₄)(NH₂) ionic liquid reached 6 × 10⁻² S/cm. For Li₄(BH₄)(NH₂)₃, the activation energy was 0.26 eV before melting (513 K) and suggested higher mobility of Li⁺ ions than in HT-LiBH₄ (0.53 eV) and Li₂(BH₄)(NH₂) [Reference Matsuo and Orimo11, Reference Matsuo, Remhof, Martelli, Caputo, Ernst, Miura, Sato, Oguchi, Maekawa, Takamura, Borgschulte, Zuttel and Orimo20]. Later, Yan et al. demonstrated that the mechanochemically processed powder mixture of LiBH₄ and LiNH₂ (molar ratio 1:2) formed cubic Li₃(BH₄)(NH₂)₂, isostructural to Li₄(BH₄)(NH₂)₃, whose σ was 6.4 × 10⁻³ S/cm at 313 K [Reference Yan, Kühnel, Remhof, Duchêne, Reyes, Rentsch, Łodziana and Battaglia22]. Recently, a new orthorhombic Li₅(BH₄)₃NH phase was obtained; however, its lithium ionic conductivity at RT was only 10⁻⁶ S/cm [Reference Wolczyk, Paik, Sato, Nervi, Brighi, GharibDoust, Chierotti, Matsuo, Li, Gobetto, Jensen, Cerny, Orimo and Baricco23].

LiY(BH₄)₄ is one of the few bimetallic Li-based borohydrides that has been studied for a potential application as a SSE [Reference Roedern, Lee, Ley, Park, Cho, Skibsted and Jensen24]. This metastable tetragonal phase was synthesized by thermal treatment of ball milled precursors. The significant enlargement of the LiY(BH₄)₄ unit cell (by 14%), with respect to the constituting components, was assumed to contribute to the relatively high mobility of Li⁺ cations, which at RT was 1.26 × 10⁻⁶ S/cm. In 2013, Černý et al. reported on the Li⁺ ionic conductivity in Li₃MZn₅(BH₄)₁₅ (M = Mg, Mn), first trimetallic borhydrides [Reference Cerny, Schouwink, Sadikin, Stare, Smrcok, Richter and Jensen28]. Their complex crystal structure, which comprises channels built from face-sharing [M(BH₄)₆] octahedra, raised hope for the high mobility of lithium cations. Unfortunately, the order distribution of Li⁺ over the crystallographic sites resulted in a low σ, which at RT was only 4 × 10⁻⁸ S/cm.

A new family of mixed-cation mixed-anion borohydride chlorides, based on Li and rare-earth elements, was discovered in 2012 and later on expanded by other halide analogues (Fig. 2 and Table I) [Reference Ley, Ravnsbæk, Filinchuk, Lee, Janot, Cho, Skibsted and Jensen25, Reference GharibDoust, Brighi, Sadikin, Ravnsbaek, Cerny, Skibsted and Jensen27]. The LiM(BH₄)₃Cl series, with M = Ce, Gd, and La, was obtained by the mechanochemical synthesis and subsequent heat treatment of MCl₃ and LiBH₄ powder mixtures. The PXD studies revealed that the compounds represented a new type of the crystal structure with isolated tetranuclear ionic clusters [M₄Cl₄(BH₃)₁₂]⁴⁻ that were charge-balanced by Li⁺ cations. LiM(BH₄)₃Cl were found to be large entropy-stabilized compounds, with the disordered distribution of Li-ions, which boosted the ionic conductivity of compounds. At 293 K, the reported σ values were 1.02 × 10⁻⁴, 3.5 × 10⁻⁴, and 2.3 × 10⁻⁴ S/cm for Ce-, Gd-, and La-containing compounds, respectively. In another study, the series of LiLa(BH₄)₃X, X = Br, Cl, and I, was synthesized by the addition reaction, which increased the purity of the samples [Reference GharibDoust, Brighi, Sadikin, Ravnsbaek, Cerny, Skibsted and Jensen27]. At 293 K, the ionic conductivity showed the following trend: LiLa(BH₄)₃Br > LiLa(BH₄)₃I > LiLa(BH₄)₃Cl and oscillated around 10⁻⁵ S/cm. The results indicated a possible influence of the halide size on the mobility of Li-ions. The topological analysis of conduction pathways suggested a channel with two various structural apertures, crossed by Li⁺ while diffusing through vacancies. The size of these windows narrowed with an increasing size of the halide ion and was optimized for the structure of the bromide-based compound. It was found that the ionic conductivity of LiLa(BH₄)₃Cl was lower (1.09 × 10⁻⁵ S/cm) than the values reported earlier, for the same compound obtained by a metathesis reaction, and resulted possibly from the percolation effect [Reference Bunde, Dieterich and Roman112], already observed in mechanochemically processed materials. The study also indicated a correlation between the halide ion type and activation energy, which was the lowest for LiLa(BH₄)₃I and the highest for LiLa(BH₄)₃Cl.

Figure 2: Ionic conductivity of Li⁺ in selected light-metal complex hydrides and boron clusters. Reprinted with permission from Ref. Reference Paskevicius, Jepsen, Schouwink, Cerny, Ravnsbaek, Filinchuk, Dornheim, Besenbacherf and Jensen8, copyright 2017 The Royal Society of Chemistry.

The newly discovered bimetallic LiCa₃(BH₄)(BO₃)₂ borohydride borate [Reference Lee, Filinchuk, Lee, Suh, Kim, Yu and Cho113] was also reported to have a relatively high ionic conductivity at RT (2.5 × 10⁻⁶ S/cm) [Reference Didelot and Cerny31]. This number was further increased to 1 × 10⁻⁵ S/cm for the samples prepared with excess of lithium and doped with either heterovalent Na⁺ or homovalent Sr²⁺. The presence of extra atoms in the crystal structure did not affect the activation energy values, which indicated the same conduction mechanism in the undoped and doped compositions. The topological analysis suggested that the conduction paths were composed exclusively of BO₃³⁻ anions, which formed face connected tetra- and octahedra accessible for Li⁺ jumps. The percolating pathway was stabilized by the calcium borohydride substructure, which was not directly involved in the conduction mechanism and indicated vacancy-dependent mobility of Li-ions. LiCa₃(BH₄)(BO₃)₂, as a first member of a new family of mixed anion hydrides, has opened possibility for the formation of other hydride-oxide compositions, e.g., SO₄²⁻, PO₄³⁻, or PS₄³⁻.

The investigation of the ionic conduction in borohydrides showed that in this group of compounds, the phenomenon is not exclusively vacancy-dependent and can be explained by a “paddle-wheel” mechanism [Reference Lee, Ley, Jensen and Cho26, Reference Martelli, Remhof, Borgschulte, Ackermann, Strassle, Embs, Ernst, Matsuo, Orimo and Zuttel114, Reference Skripov, Soloninin, Ley, Jensen and Filinchuk115]. It means that in the crystal structures based on [M′X_n]^δ− polyanions, the fast ionic conductivity is promoted by a high rotational mobility of the [M′X_n]^δ− units, which in turn, decreases the associated activation energy [Reference Jansen116].

Owning to a high-density of ionic bonds and packed structures, which are based on coordination polyhedral, metal borohydrides and metal oxides, are very similar. The crystallographic and chemical analogies were successfully used to establish formation rules for perovskite-type metal borohydrides and to predict their physical properties [Reference Schouwink, Ley, Tissot, Hagemann, Jensen, Smrcok and Cerny117]. Disordered complex hydride perovskites may serve as ionic conductors in analogy with oxides such as La_2/3−xLi_3xTiO₃. So far, only limited number of these compounds have been explored for their potential application in electrochemical energy storage solutions. Cs₂LiY(BH₄)₆, a trimetallic borohydride, is one of the studied examples [Reference Sadikin, Stare, Schouwink, Ley, Jensen, Meden and Cerny29]. Unfortunately, its cubic double-perovskite crystal structure possesses neither empty crystallographic sites available for Li⁺ jumps nor conduction channels, with sizes suitable for migration of lithium ions. Thus, at 303 K, its reported σ value was as low as 10⁻⁹ S/cm. The Li-ion conductivity of doped lanthanide-oxide Li_3+xLa₃M₂O₁₂ garnets led to the study on the alkali rare-earth garnet borohydrides, of which Li₃K₃M₂(BH₄)₁₂, M = Ce, La, were the first reported examples [Reference Brighi, Schouwink, Sadikin and Cerny30]. Their ionic conductivity at RT, 3 × 10⁻⁷ and 6 × 10⁻⁷ S/cm, respectively, was 7 orders of magnitude higher than that of undoped metal oxide garnet analogues. The doping effect of mono- and divalent cations on the Li-ion conductivity in these compounds was also investigated. The results suggested that the presence of Sr²⁺ or Eu²⁺ in Ce- and La-containing compositions increased σ by one order of magnitude in the measured temperature range. Interestingly, to observe this effect, dopant concentrations over four times smaller than in oxides were needed. The enhancement was likely due to changes in the structural dynamics related to the disordering of [BH₄]⁻ tetrahedra.

The new concept of utilizing reversible ammonia gas sorption in LiBH₄ for formation of structural defects, which consequently enhanced the Li-ion mobility, has been recently reported by Zhang et al. (Fig. 3 and Table I) [Reference Zhang, Wang, Song, Miyaoka, Shinzato, Miyaoka, Ichikawa, Shi, Zhang, Isobe, Hashimoto and Kojima32, Reference Mohtadi118]. Solid-state lithium borohydride ammoniates Li(NH₃)_xBH₄, x = 0.5 and 1.0, showed a drastic change in the ionic conductivity due to the structural modifications, resultant from the desorption of NH₃. For Li(NH₃)BH₄, the σ value increased from 1.5 × 10⁻⁶ S/cm at 303 K to 2.21 × 10⁻³ S/cm at temperatures >313 K, which was associated with the formation of Schottky defects in the compound crystal structure. These new solid-state superconductors were also characterized by a wide electrochemical window (4 V versus Li/Li⁺).

Figure 3: (a) Crystal structures of mono-ammoniate (i) and LiBH₄ (ii) with marked unit cells. (b) Li-ion conductivity of Li(NH₃)BH₄ during the heating (black) and cooling processes (red). The blue line represents σ of the as-received LiBH₄. (c) Status of the ionic conductivity in several borohydride-type electrolytes. Reprinted with permission from Ref. Reference Mohtadi118, copyright 2018 Elsevier Inc.

Composites

One of the alternative methods promoting the high ionic conductivity in lithium borohydride is the formation of composites with other materials. High σ (10⁻⁴ S/cm) at RT was observed for the LiBH₄–SiO₂ nanocomposite prepared by melt infiltration of the borohydride into a mesoporous inorganic silica scaffold [Reference Blanchard, Nale, Sveinbjornsson, Eggenhuisen, Verkuijlen, Suwarno, Vegge, Kentgens and de Jongh33, Reference Ngene, Adelhelm, Beale, de Jong and de Jongh119]. Given the high concentration of the insulating SiO₂ in the material (up to 50 vol%) and limited stabilization of HT-LiBH₄ in the matrix pores, the measured ionic conductivity values were surprising. However, the study suggested that the high mobility of Li⁺ in this case was related to the high density of defects and low diffusion barriers at the interface between two solids, which resulted from disorder, strain, and space-charge regions [Reference Blanchard, Nale, Sveinbjornsson, Eggenhuisen, Verkuijlen, Suwarno, Vegge, Kentgens and de Jongh33, Reference Verdal, Udovic, Rush, Liu, Majzoub, Vajo and Gross120]. The follow up studies on the same system, though synthesized by high energy milling, also demonstrated the fast Li⁺ diffusion in the obtained materials (10⁻⁴ S/cm at RT) [Reference Choi, Lee, Oh and Cho34]. This confirmed the importance of the interface-related processes on the enhancement of the ionic conductivity in LiBH₄–SiO₂. A similar study was later performed on the LiBH₄–Al₂O₃ system [Reference Choi, Lee, Choi, Chae, Oh and Cho35]. The results showed that the Li⁺ ion conductivity of the composite reached 2 × 10⁻⁴ S/cm at RT and was associated with the formation of a highly defective interface between two solids. Motivated by this discovery, other nano/composites have also been explored. Teprovich et al. reported on fast ionic conduction in the LiBH₄–C₆₀ system and demonstrated the enhanced mobility of Li⁺ in the presence of C₆₀, which for the mixture with a ratio of 70:30 wt% reached 2.0 × 10⁻⁵ and 2.0 × 10⁻³ at 298 K and 413 K, respectively [Reference Teprovich, Colon-Mercado, Ward, Peters, Giri, Zhou, Greenway, Compton, Jena and Zidan36]. This behavior resulted from the nonionic mechanism, which assumed destabilization of the Li⁺/[BH₄]⁻ ion pairs in the presence of fullerenes.

The high ionic conductivity of polycrystalline α- and β-Li₃N [Reference Li, Wu, Araújo, Scheicher, Blomqvist, Ahuja, Xiong, Feng and Chen121] at RT (approximately 10⁻⁴ S/cm for both phases), with however a narrow potential window (<1 V), motivated studies on the LiBH₄–Li₃N system [Reference Zhan, Zhang, Zhuang, Fang, Zhu, Guo, Chen, Wang and Li37]. Materials prepared by mechanochemical methods contained crystalline LT-LiBH₄ and β-Li₃N, and below 380 K, their ionic conductivity was two orders of magnitude higher than unprocessed LT-LiBH₄ (10⁻⁵ versus 10⁻⁷ S/cm at 323 K). Although no presence of crystalline intermediate phases was observed, IR studies indicated the formation of N–H chemical bonds. In addition, no occurrence of hexagonal HT-LiBH₄ was confirmed in the studied samples. Based on the results, it was concluded that the observed increase in the mobility of Li⁺ ions was most likely associated with a high defect concentration at the interfaces between crystalline structures and amorphous components. A high σ of lithium imide [Reference Boukamp and Huggins48] has also attracted attention to the LiBH₄–Li₂NH system [Reference Zhang, Zhan, Zhuang, Zhu, Wan, Guo, Chen, Wang and Li38, Reference Wang, Cao, Zhang, Chen, Wu, Pistidda, Ju, Zhou, Wu, Etter, Klassen, Dornheim and Chen39]. The series of borohydride-imide composites prepared with molar ratios 1:1, 1:2, and 1:4 showed ionic conductivities of 10⁻⁶ and 10⁻² S/cm at 323 K and 373 K, respectively [Reference Zhang, Zhan, Zhuang, Zhu, Wan, Guo, Chen, Wang and Li38]. The observed enhancement of the Li-ion diffusion was due to the formation of favorable boundaries by both phases. The increasing concentration of Li₂NH in the system lowered the activation energy in the higher temperature range (373–393 K) but played an opposite effect within the lower temperature zone (323–363 K). In a recent study, the same system showed the formation of a new cavity-rich orthorhombic phase with yet unknown crystal structure [Reference Wang, Cao, Zhang, Chen, Wu, Pistidda, Ju, Zhou, Wu, Etter, Klassen, Dornheim and Chen39]. The σ values measured for this material oscillated in the same range as in the previous reports and showed the same temperature dependence as the earlier investigated compositions [Reference Zhang, Zhan, Zhuang, Zhu, Wan, Guo, Chen, Wang and Li38].

There are also several reports on the ionic conductivity of Li⁺ in borohydride–borohydride/hydride composites. A series of LiBH₄ and Ca(BH₄)₂ powder mixtures were obtained by ball milling [Reference Sveinbjörnsson, Blanchard, Myrdal, Younesi, Viskinde, Riktor, Norby and Vegge40]. The synthesized materials did not form a bimetallic borohydride. Instead, they consisted of orthorhombic LiBH₄, various Ca(BH₄)₂ polymorphs, and a small amount of CaH₂ that formed upon heating. At 313 K, ionic conductivities of the obtained composites were lower than that of LiBH₄ processed in the same way (e.g., 8.8 × 10⁻⁶ versus 4.6 × 10⁻⁵ for 0.75LiBH₄–0.25Ca(BH₄)₂ and lithium borohydride, respectively). The increased σ was linked to the formation of defects and interfaces upon milling that was likely to open new Li-ion condition pathways. The similar interpretation of the results was given for the mechanochemically processed xLiBH₄–NaBH₄ powder mixtures [Reference Xiang, Zhang, Zhan, Zhu, Guo, Chen, Wang and Li41]. Milling did not cause the formation of a stoichiometric bimetallic borohydride or a solid solution, and the obtained samples consisted of LT-LiBH₄ and NaBH₄. Below the LiBH₄ transition temperature, the composites revealed ionic conductivities of one or two orders of magnitude higher than that of pristine LT-LiBH₄. On the other hand, above 373 K, their σ reached 10⁻² S/cm. The result demonstrated that while higher amount of LiBH₄ in the mixtures increased the mobility of Li⁺, the higher fraction of NaBH₄ reduced the activation energy values. The ball milled LiBH₄–MgH₂ composite comprised LT-LiBH₄ and MgH₂ and had 10 times increased mobility of Li⁺ at temperatures <373 K, as compared with LT-LiBH₄ [Reference López-Aranguren, Berti, Dao, Zhang, Cuevas, Latroche and Jordy42]. In the follow up study, Xiang et al. investigated mechanochemically processed powder mixtures of LiBH₄–NaBH₄–xMgH₂, with x = 10, 20, and 30% [Reference Xiang, Zhang, Zhu, Guo, Chen and Li43]. The processed materials consisted of crystalline NaBH₄ and MgH₂, while the LiBH₄ phase was most likely amorphous. For the LiBH₄–NaBH₄–30%MgH₂ composition, Li⁺ ion conductivities reached 2.1 × 10⁻⁵ S/cm and 11.2 × 10⁻³ S/cm at 333 K and 383 K, respectively. Both values were significantly higher than those of LT/HT-LiBH₄. The measurements also identified wide potential windows for the studied compositions, ranging from −1 to 4 V (versus Li/Li⁺), which indicated a high electrochemical stability of the materials. Recently, the ionic conductivities of the LiBH₄–NaX (X = Cl, I) composites, obtained by ball milling, were reported [Reference Xiang, Zhang, Lin, Zhu, Guo, Chen and Li44]. The study showed the presence of the LiBH_4−xCl_x halide solid solution and the eutectic LiBH₄–NaBH₄ composite in the processed LiBH₄–NaCl powder mixture. Their coexistence was likely to facilitate the ionic transportation within the system, which below 373 K was greater by 10–100 times than that of LT-LiBH₄ and increased to values of 10⁻² S/cm above 373 K. Much higher conductivity of Li⁺ at lower temperatures (approximately 10⁻³ S/cm at 300 K) was observed in the crystalline mixture of 90LiBH₄–10P₂S₅ [Reference Unemoto, Wu, Udovic, Matsuo, Ikeshoji and Orimo45]. Thermal heating of the material did not reveal any phase transition which confirmed the composite stability up to 473 K. This new phase was characterized by high plasticity and possessed a wide potential window (0–5 V), which made it suitable for a battery application.

The addition effect of crystalline LiBH₄ on the ionic conductivity of sulfide glass electrolytes was also studied [Reference Yamauchi, Sakuda, Hayashi and Tatsumisago46]. The results showed that a higher content of LiBH₄ increased the conductivity of Li⁺ in the mechanochemically processed mixture of xLiBH₄–(100 − x)0.75Li₂S·0.25P₂S₅. At RT, the composite with x = 33 showed the highest sigma (1.6 × 10⁻³ S/cm) σ and the lowest activation energy (0.30 eV). The Raman spectroscopy results suggested that the state of [BH₄]⁻ incorporated into the sulfide glass matrix was the same as in the HT-LiBH₄, with the high rotational freedom and delocalized negative charge. As a consequence, the electrostatic interactions between Li⁺ and [BH₄]⁻ were weakened, which increased σ values in the obtained glasses. The addition of Li(BH₄)₃I also enhanced the Li-ion ion conduction of 0.75Li₂S·0.25P₂S₅ [Reference El Kharbachi, Hu, Yoshida, Vajeeston, Kim, Sorby, Orimo, Fjellvag and Hauback47]. The Li(BH₄)₃I–0.75Li₂S·0.25P₂S₅ mixture, with a molar ratio of 1:2, revealed σ in the order of 10⁻³ S/cm at RT, with the activation energy of 0.30 eV.

LIBs

The first example of the LiBH₄ application as SSE in all-solid-state battery was reported in 2013 by Takahashi et al. [Reference Takahashi, Hattori, Yamazaki, Takada, Matsuo, Orimo, Maekawa and Takamura77]. The compound was tested in an electrochemical cell with Li as an anode and LiCoO₂ as a positive electrode. The battery performance was evaluated at 393 K, which ensured the stability of the highly conductive HT-LiBH₄ phase. Although SSE showed a high compatibility with the Li electrode, at the same time, the reaction between LiBH₄ and LiCoO₂ occurred. As a result, a high interfacial resistance was observed, which in turn led to the significant capacity loss. To minimize this effect, an extra layer of amorphous Li₂PO₄ was introduced between SEE and the cathode material. This approach improved the battery cycle performance as it retained 97% of the initial discharge capacity (89 mA h/g) after 30 cycles. Sveinbjörnson et al. proposed an alternative approach and showed that coating was unnecessary for a cathode with a lower redox potential [Reference Sveinbjörnsson, Christiansen, Viskinde, Norby and Vegge78]. The study investigated the performance of SSB with Li₄Ti₅O₁₂ (LTO) as a positive electrode. Lithium titanite has a redox potential of 1.55 V (versus Li/Li⁺), which is lower than that of LiCoO₂ (3.9 V versus Li/Li⁺). The application of the LiBH₄–LiI solid solution as SSE decreased the cell operation temperature to 333 K. The initial discharge capacity of the battery was 277 mA h/g and accounted for 81% of a LTO theoretical value. This was comparable with the results obtained for the same battery with a liquid electrolyte. Although the all-solid-state battery delivered the discharge capacity above 110 mA h/g (65% of a LTO utilization ratio) during the initial 10 cycles, its discharge capacity retention was worse than that of the liquid electrolyte counterpart. The increased resistance in the SSB cell, due to insufficient contact between SSE and LTO and/or formation of a passivation layer at their interface, was blamed for the observed capacity loss. In a later study, Yoshida et al. replaced the pure LTO cathode with a composite electrode, which consisted of homogeneously dispersed LTO/Li₄(BH₄)₃I with conductive carbon additives [Reference Yoshida, Suzuki, Kawaji, Unemoto and Orimo79]. The highly deformable nature of the Li₄(BH₄)₃I SSE resulted in the formation of a tight interfacial contact with the cathode and resulted in the stable battery operation in the range of 296–423 K. At the highest temperature, the battery delivered discharge capacities of 170 and 158 mA h/g, during the first and second cycles, which corresponded to 97% and 90% of a LTO utilization ratio, respectively. The battery capacity retention at this temperature was also very high, and after 100 cycles, it accounted for 140 mA h/g. At 296 K, the battery delivered 122 mA h/g during the initial discharge and its capacity retention was 91% after the fifth cycle. Regardless of the operation temperature, the Coulombic efficiency was nearly 100%, which indicated that the charge–discharge cycling proceeded without side reactions. A smooth operation of the cell was possible due to very limited charge transfer resistance at the SSE/cathode interface, resulting from a tight contact among electrode constituents. The successful demonstration of the all-solid-state LIB with Li₄(BH₄)₃I at RT (298 K) was also reported by Unemoto et al. The cell comprised the LiNbO₃-coated LiCoO₂ and 80Li₂S·20P₂S₅ (LPS) composite electrode, and the Li anode [Reference Unemoto, Nogami, Tazawa, Taniguchi and Orimo19]. At the C-rate of 0.1, the discharge capacity was reduced from 92 to 82 mA h/g between the 1st and 20th cycle and the capacity retention was 90%. Although the Coulombic efficiency was only 75% in the initial cycle, it recovered to almost 100% after the second one. The stable and repeated battery operation was possible exclusively by combining SSE electrolytes with different electrochemical stabilities. Another example of the application of Li₄(BH₄)₃I as SSE in LIB was reported by Suzuki et al. [Reference Suzuki, Kawaji, Yoshida, Unemoto and Orimo80]. The tested battery consisted of a composite cathode, with LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) as an active material and TiO₂-doped Li₃BO₃, to avoid a direct contact between NMC and SSE. An additional LiBH₄ + xLiNH₂ (x = 1, 2) adhesive layer was introduced between the positive electrode and Li₄(BH₄)₃I. LiBH₄ + xLiNH₂ has been reported to have the high Li-ion conductivity and a melting temperature of 323 K. Thus, by removing the surface roughness and imperfections, the compound was expected to improve the interface compactness and suppress the contact resistance. The study performed at 423 K demonstrated that the presence of the adhesive layer increased the first discharge capacity from 56 mA h/g to 114 mA h/g, in battery without and with LiBH₄ + xLiNH₂, respectively. However, it also increased the initial charge capacity above the theoretical value (234 mA h/g versus 115 mA h/g), which suggested the occurrence of side reactions. The presence of the adhesive layer also improved the cycling performance as the capacity retention increased from 29% to 71% for the battery without and with LiBH₄ + xLiNH₂, respectively.

The feasibility of HT-LiBH₄ as SSE was also tested in the lithium–sulfur (Li–S) conversion batteries, characterized by high energy density [Reference Unemoto, Yasaku, Nogami, Tazawa, Taniguchi, Matsuo, Ikeshoji and Orimo81]. To enhance the electrochemical activity of insulating elemental S, a composite S/LiBH₄ cathode with conductive carbon (C) was prepared by ball milling. The results demonstrated stable battery operations at 393 K and underlined the important role of borohydrides’ ductility, which ensured a good contact at the SSE–cathode interface, obtained exclusively by cold-pressing. The proposed Li–S cell configuration delivered 1140 mA h/g at the rate of 0.05C, during the first discharge, which corresponded to 70% of a sulfur utilization ratio. Over the next 45 cycles, the discharge capacity remained as high as 730 mA h/g with nearly 100% Coulombic efficiency. The Li–S battery operation with the S/C composite cathode was also evaluated with the LiBH₄–LiCl solid solution as SSE [Reference Unemoto, Chen, Wang, Matsuo, Ikeshoji and Orimo82]. The homogeneous dispersion of the electrode constituents ensured a very good SSE/electrode contact and allowed for the reversible battery operation at 373 K, with the discharge capacity of 1377 mA h/g and 636 mA h/g during the initial and the fifth cycle, respectively. To further lower the operation temperatures of batteries, Das et al. applied nanoconfined LiBH₄ as SSE in the Li–S cell [Reference Das, Ngene, Norby, Vegge, de Jongh and Blanchard83]. The battery containing S/C and Li as positive and negative electrodes, respectively, was evaluated at 328 K and various charge–discharge rates (0.03–0.12C). At higher C-rates, a higher drop of the C/S electrode potential was observed, which obstructed the efficient sulfur utilization. However, by lowering the discharge rate, a quick recovery of the full capacity was obtained. Over 40 cycles, the cell showed good performances and maintained the discharge capacity as high as 1220 mA h/g at 0.03C. Lithium borohydride SSE demonstrated high compatibility also with an all-solid-state metal hydride–sulfur LIB [Reference López-Aranguren, Berti, Dao, Zhang, Cuevas, Latroche and Jordy42]. The cell with Li₂S/LiBH₄ as a cathode, 0.8MgH₂–0.2TiH₂/LiBH₄ as an anode, and LiBH₄ as SSE was operated at 393 K. The reversible specific capacity reached 910 mA h/g, and after 25 charge–discharge cycles, the battery retained 85% of the initial capacity value.

Recently, the stability of LiCe(BH₄)₃Cl as SSE in a Li–S battery was investigated [Reference Nguyen, Fleutot and Janot84]. The compound was tested in a cell with a S/LiCe(BH₄)₃Cl composite as a positive electrode and the LiIn alloy as the negative one. The reversible reaction between Li and S was observed at 318 K and a rate of C/100. Although the capacity of the initial discharge and charge reached 1196 mA h/g (71% of the theoretical value) and 879 mA h, respectively, subsequent cycling resulted in significant capacity loss. Still, the capacity retention was satisfactory with the discharge capacity of 510 mA h/g after the ninth cycle.

Much better results and a stable interface formation between the HT-LiBH₄ SSE and the active cathode material were reported for the TiS₂–Li cell (Fig. 7 and Table II) [Reference Unemoto, Matsuo and Orimo12, Reference Unemoto, Ikeshoji, Yasaku, Matsuo, Stavila, Udovic and Orimo85]. A solid-state battery with a TiS₂/LiBH₄ composite positive electrode, LiBH₄ and Li, as the electrolyte and the anode, respectively, was successfully cycled 300 times at 393 K and a rate of 0.2C. Although the initial discharge capacity was only 80 mA h/g (versus 239 mA h/g of the theoretical value), at the second cycle, the parameter reached 205 mA h/g with 100% Coulombic efficiency. This corresponded to 85% of a TiS₂ utilization ratio. The battery did not reveal significant capacity fading and during the final discharge the cell delivered 180 mA h/g with the same Coulombic efficiency as in the second cycle. This good durability was explained by the chemical/electrochemical oxidation of the borohydride just below the TiS₂ surface. This was accompanied by the hydrogen release and the new phase formation, most likely Li₂B₁₂H₁₂, with high ionic conductivity. The compound revealed high oxidative stability toward LiBH₄ and a charge transfer reactivity with the Li anode and as such was likely to act as a stable interfacial layer that enabled numerous charge–discharge cycles.

Figure 7: Discharge–charge profiles of the bulk-type all-solid-state TiS₂/Li battery operated at 393 K and 0.2C (a). Graphical representation of the Li₂B₁₂H₁₂ formation at the TiS₂/Li interface as a result of the chemical/electrochemical reaction between LiBH₄ and H₂ (b). A photograph of the bulk-type all-solid-state TiS₂/Li battery (c). A part of the Li negative electrode was delaminated for clarity of the battery configuration. Reprinted with permission from Ref. Reference Unemoto, Ikeshoji, Yasaku, Matsuo, Stavila, Udovic and Orimo85, copyright 2015 American Chemical Society.

The oxidative stability of Li₄(BH₄)₃I as SSE was evaluated based on the performance of TiS₂–Li batteries with the TiS₂/LiBH₄ and TiS₂/Li₄(BH₄)₃I composite electrodes [Reference Unemoto, Nogami, Tazawa, Taniguchi and Orimo19]. Both cells were tested at 393 K with the discharge–charge rate of 0.05C. Similar to HT-LiBH₄, the I⁻-substituted LiBH₄ preserved the high mechanical plasticity, which ensured the formation of a robust and tight SSE/cathode interface. Due to the self-discharge reaction and formation of Li_xTiS₂, the initial discharge capacity of the TiS₂/LiBH₄-based battery was 61 mA h/g but recovered to 216 mA h/g in the second cycle. On the other hand, the first charge capacity reached 419 mA h/g, which was far above the theoretical value and resulted from the oxidative decomposition of unreacted lithium borohydride. The battery was characterized by a high capacity retention (20 mA h/g) after the 15th cycle. In the cell with the TiS₂/Li₄(BH₄)₃I electrode, the first discharge capacity was only 49 mA h/g and recovered to 141 mA h/g during the second cycle. This low initial capacity value was due to the reaction between TiS₂ and Li₄(BH₄)₃I, which caused the battery self-discharge prior testing. Over the next 15 cycles, the cell capacity dropped significantly, which was correlated with the steady increase in the internal battery resistance and the lower interface stability between TiS₂ and Li₄(BH₄)₃I than between TiS₂ and LiBH₄. The integration of the crystalline 90LiBH₄·10P₂S₅ (LPS) mixture as SSE in the TiS₂–Li battery allowed for its reversible operation at 300 K [Reference Unemoto, Wu, Udovic, Matsuo, Ikeshoji and Orimo45]. The battery was tested against the InLi alloy anode and operated at the C-rate of 0.1. The first and second discharge capacity reached 192 and 228 mA h/g, respectively. After the 10th cycle, the capacity retention was 98% with almost 100% Coulombic efficiency. Recently, a mixture of Li₄(BH₄)₃I and LPS was utilized as SSE in the same battery type [Reference El Kharbachi, Hu, Sorby, Maehlen, Vullum, Fjellvag and Hauback86]. The TiS₂/Li₄(BH₄)₃I–LPS composite electrode was tested with the Li₄(BH₄)₃I–LPS electrolyte and Li as a counter electrode at 303 K. The results demonstrated the high compatibility of this SSE mixture toward the electrochemically active materials. The capacity of the initial discharge corresponded to 89% of the theoretical one. The lower C-rate ensured better battery cyclability with a higher Coulombic efficiency.

As soon as metal hydrides and complex transition metal hydrides were proposed as conversion-type anode materials in LIBs, due to their high theoretical Li storage capacity, low volume expansion, and suitable working potential, the possibility of designing SSB with LiBH₄ as SSE has been explored [Reference Mohtadi and Orimo6, Reference Oumellal, Rougier, Nazri, Tarascon and Aymard144, Reference Aymard, Oumellal and Bonnet145]. It has been shown that electrochemical performance of the MgH₂- and TiH₂-based composite electrodes was much better with LiBH₄ as SSE than with conventional organic liquid electrolytes [Reference Zeng, Kawahito, Ikeda, Ichikawa, Miyaoka and Kojima87, Reference Zeng, Kawahito and Ichikawa88, Reference El Kharbachi, Uesato, Kawai, Wenner, Miyaoka, Sørby, Fjellvåg, Ichikawa and Hauback90]. The results for LIB with the TiH₂/LiBH₄ composite electrode, operated at 393 K, showed that the Li insertion capacity faded little, from 1094 to 878 mA h/g between the 2nd and 50th charge–discharge cycle [Reference Zeng, Kawahito and Ichikawa88]. For SSB with the composite MgH₂/LiBH₄ and MgH₂/LiBH₄/Nb₂O₅ anodes, the initial discharge capacity was 1575 mA h/g and 1650 mA h/g at the C-rate of 0.8, which corresponded to 94% and 95% Coulombic efficiency, respectively. The reported numbers were lower than the theoretical value (2038 mA h/g), but much higher than those obtained for their counterparts with the liquid electrolyte. In addition, both SSBs had improved cyclic stability and better capacity retention. The battery with the carbon nanofiber supported MgH₂/LiBH₄ electrode and LiBH₄ SSE was also reported to perform better than the same battery with LPS glass-type SSE [Reference Zeng, Ichikawa, Kawahito, Miyaoka and Kojima89]. The battery with LPS delivered an initial discharge capacity of 1214 mA h/g; however, within next 15 cycles, the value dropped rapidly to only 100 mA h/g. The first charge capacity reached only 575 mA h/g. In comparison, the battery with LiBH₄ SSE showed an initial discharge capacity of 1728 mA h/g (94% Coulombic efficiency), which faded slowly with the increased cycle numbers and remained as high as 1017 mA h/g (99.5% Coulombic efficiency) after 50 cycles [Reference Zeng, Ichikawa, Kawahito, Miyaoka and Kojima89]. The suitability of LiBH₄ as SSE was also demonstrated for all-solid-state LiB with the MgH₂/CoO₂ composite electrode [Reference El Kharbachi, Uesato, Kawai, Wenner, Miyaoka, Sørby, Fjellvåg, Ichikawa and Hauback90]. The studies concluded that in this battery-type, solid LiBH₄ promoted the hydride conversion reaction due to its dual effect on the Li⁺ and H⁻ conductivity. Additionally, a crucial role of the specific anode/electrolyte interface on the reaction kinetic was suggested [Reference Zeng, Kawahito, Ikeda, Ichikawa, Miyaoka and Kojima87, Reference Zeng, Kawahito and Ichikawa88, Reference Zeng, Ichikawa, Kawahito, Miyaoka and Kojima89]. Recently, the RT performance of conversion-type MgH₂ anode with a mixture of Li₄(BH₄)₃I and LPS as SEE, and lithium as a counter electrode, was reported and compared with the liquid electrolyte cell [Reference El Kharbachi, Hu, Sorby, Maehlen, Vullum, Fjellvag and Hauback86]. The liquid electrolyte-based battery showed higher discharge capacity, but SSB demonstrated a higher reversibility yield. Another recent example of the LiBH₄ SSE application in hydride-based LiBs was reported for the Mg₂FeH₆-based conversion type anode [Reference Huen and Ravnsbæk91]. In comparison to a cell with a liquid electrolyte, this SSB exhibited a higher value of the initial discharge capacity, 1254 versus 1100 mA h/g, and significantly better Coulombic efficiency, 68 versus 27%. Also, after the 10th cycle, the retained capacity of the cell with the SSE electrolyte was above 3 times higher than that of the conventional counterpart.

The application of LiBH₄ as SSE was reported for all-solid-state LIB with an aluminum composite anode (Al/LiBH₄) [Reference Weeks, Tinkey, Ward, Lascola, Zidan and Teprovich92]. The cell operated at 408 K and the C-rate of 0.1 revealed the reversible capacity of 895 mA h/g during the initial cycle, which was close to the theoretical value (993 mA h/g). However, subsequent cycling of the material led to a significant capacity loss. Evaluation of the results obtained for charge–discharge cycles at various C-rates indicated that lithium borohydride could facilitate the reversible lithiation of aluminum up to 1C.

Recently, lithium borohydride as SSE was used in a battery cell with a nanostructured Bi₂Te₃/LiBH₄ composite anode and Li as an active cathode material [Reference Singh, Kumari, Rathore, Shinzato, Ichikawa, Verma, Saraswat, Awasthi, Jain and Kumar93]. The electrochemical performances of SSBs with Bi₂Te₃ nanoparticles and nanosheets were tested. The initial discharge–charge capacities at a rate of 0.1C were 550 mA h/g and 540 mA h/g for nanoparticles and nanoplates-based batteries, respectively.

The suitability of the LiBH₄–C₆₀ nanocomposite as SSE in all-solid-state LIB was tested in a cell with a silicon/conductive carbon anode and the Li cathode, at 353 K and C-rate of 0.1C [Reference Teprovich, Colon-Mercado, Ward, Peters, Giri, Zhou, Greenway, Compton, Jena and Zidan36]. The first discharge capacity reached 91% of the theoretical value (4200 mA h/4 for Li_4.4Si); however, only two charge–discharge cycles were performed due to the significant performance degradation. This was attributed to the conductivity loss in Si as a result of its large volume expansion upon cycling. The same SSE was also tested in a cell with LiCoO₂ and Li, as a cathode and an anode, respectively [Reference Teprovich, Colon-Mercado, Ward, Peters, Giri, Zhou, Greenway, Compton, Jena and Zidan36]. The battery was cycled five times with a rate of 0.1C at 323 K and 353 K. The reversible capacity values reached only 8 and 10 mA h/g, respectively, and were explained by either poor contacts between SSE and the active material and/or side reactions between the electrolyte and LiCoO₂.

Battery demonstrations were also reported for boron cluster-based electrolytes using different cathodes. These included battery cycling in H/B deficient Li₂B₁₂H₁₂ at 353 K, where the Li metal anode and TiS₂ (voltage approximately 2.6 V) were the negative and positive electrodes, respectively [Reference Kim, Toyama, Oguchi, Sato, Takagi, Ikeshoji and Orimo52]. The battery was cycled at low rates (0.05C) and showed a gradual capacity fade. The TiS₂ cathode was also used to demonstrate the performance of the LiCB₁₁H₁₂ battery operated at 403 K and the C-rate of 0.2. As for Li₂B₁₂H₁₂, also in this case, capacity fading was observed (i.e., the capacity dropped from 240 to 180 mA h/g by the fifth cycle). Batteries were also examined with higher voltage cathodes. The ball milled Li₂B₁₂H₁₂ was used in a Li/LiCoO₂ cell, which was run at 323 K and a rate of 0.2C. However, only 40% of the initial capacity was obtained by the 20th cycle.

Na-ion batteries

The Na₃NH₂B₁₂H₁₂ complex hydride, obtained from metal amide and higher borane, has been recently employed as SSE in the all-solid-state NIB, with the composite TiS₂/Na₃NH₂B₁₂H₁₂ cathode and the Na anode [Reference He, Lin, Li, Filinchuk, Zhang, Liu, Yang, Hou, Deng, Li, Shao, Wang and Lu61]. The cell was reversibly operated at 535 K and a rate of 0.1C, over 200 cycles. During the second discharge, the battery delivered 146 mA h/g with almost 100% Coulombic efficiency, which indicated no significant side reactions. After 100 cycles, the capacity retention was as high as 102 mA h/g and dropped to 77 mA h/g during subsequent 100 cycles. Such behavior was likely due the partial decomposition of Na₃NH₂B₁₂H₁₂.

One of the most successful demonstration of the Na-ion battery was by employing Na(B₁₂H₁₂)_0.5(B₁₀H₁₀)_0.5 in a 3 V Na metal cell, which was operated for 250 cycles (Fig. 8 and Table II). This example has represented an important validation on the utility of hydroborates as practical SSEs for relatively high voltage Na ion batteries (Fig. 6) [Reference Duchene, Kuhnel, Stilp, Cuervo Reyes, Remhof, Hagemann and Battaglia99]. The key to this successful demonstration was a good contact between the cathode (NaCrO₂, theoretical capacity of 120 mA h/g) and the electrolyte, accomplished by impregnating the cathode particles with the electrolyte. This was achieved by dissolving the latter in anhydrous methanol and dispersing NaCrO₂ into the solution, followed by drying in a vacuum and heat treatment at 270 °C to recrystallize the electrolyte. This example highlighted the importance of the careful electrode design in enabling hydride-based batteries.

Figure 8: (a) Schematic and SEM cross sections of the device investigated in the 3 V Na battery showing the different components of the cell. (b) Charge/discharge profiles for the 1st, 2nd, and 20th cycle of Na|Na(B₁₂H₁₂)_0.5(B₁₀H₁₀)_0.5|NaCrO₂ cells with mixed and impregnated cathode mixtures. (c) Long term cycling of a cell using the impregnated cathode. The cell was initially activated by 3 charge/discharge cycles at 0.05C and subsequently cycled 250 times at 0.2C. Charge and discharge capacity and efficiency are shown every 2 cycles. Reprinted with permission from Ref. Reference Duchene, Kuhnel, Stilp, Cuervo Reyes, Remhof, Hagemann and Battaglia99, copyright 2017 The Royal Society of Chemistry.

Borohydrides

Meggiolaro et al. reported on the incorporation of the selected borohydrides as active anode materials in a Li-ion cell [Reference Meggiolaro, Farina, Silvestri, Panero, Brutti and Reale94]. The study spanned ball milled LiBH₄, NaBH₄, KBH₄, Mg(BH₄)₂, and Ca(BH₄)₂, which were tested against Li with a solution of LiPF₆ in the ethylene/dimethyl carbonate mixture as an electrolyte. Batteries were evaluated in a potential range of 2.5/2.0–0.01 V and at a rate of C/20. LiBH₄, KBH₄, and Ca(BH₄)₂ showed no or negligible electrochemical activity, which was likely due to limited electron/ionic conductivity of the materials and/or high activation energy of the conversion reaction. More encouraging results were obtained for NaBH₄ and Mg(BH₄)₂, for which specific capacities reached approximately 200 mA h/g upon the first reduction. The study also emphasized the importance of the relationship between the duration of mechanochemical powder processing and material reactivity. For shorter milled Mg(BH₄)₂, a higher number of redox processes in a battery was observed with the first discharge capacity reaching 540 mA h/g. Although the measured capacities were much lower than the theoretical ones, the study showed possible enhancement of the battery performances by the optimization of the active material morphology.

Alanates

The first demonstration of alanates as conversion anodes in the Li-ion cell was presented for LiAlH₄ and Li₃AlH₆ by Silvestri et al. [Reference Silvestri, Forgia, Farina, Meggiolaro, Panero, La Barbera, Brutti and Reale95]. The mechanochemically processed materials were tested in a three-electrode cell, with Li as a counter electrode and a solution of LiPF₆ as an electrolyte, at a rate of C/20, in a potential range of 2.5–0.01 V. The study demonstrated that the reversible incorporation/de-incorporation of Li as well as the occurrence of a conversion reaction in both materials pointed to a high complexity of the reaction mechanism. Even though the capacities upon discharge were as high as 1180 mA h/g and 900 mA h/g for LiAlH₄ and Li₃AlH₆, respectively (versus theoretical values: 2119 mA h/g and 1493 mA h/g), the reversibility of the processes was poor as only 16% and 22% of these values, respectively, were recovered during the subsequent charge cycle. Such behavior was associated with a large volume variation upon conversion (30%) and alloying reactions (100%). In another study, the performance of unprocessed LiAlH₄ was tested in the same electrochemical environment at C/10 [Reference Teprovich, Zhang, Colón-Mercado, Cuevas, Peters, Greenway, Zidan and Latroche148]. The study also confirmed the occurrence of a poorly reversible, conversion reaction between LiAlH₄ and Li, with an unclear reaction pathway.

The compatibility of NaAlH₄, Na₃AlH₆, and LiNa₂AlH₆ with Li electrode in a solution of LiPF₆ as an electrolyte was also explored [Reference Silvestri, Farina, Meggiolaro, Panero, Padella, Brutti and Reale96, Reference Teprovich, Zhang, Colón-Mercado, Cuevas, Peters, Greenway, Zidan and Latroche148]. While the electrochemical performances of both unprocessed and mechanochemically activated compounds were rather poor, the ball milling of NaAlH₄ with the conductive carbon showed an impressive increase in the conversion reversibility. This resulted in a much greater value of the charge capacity (1 versus 2.65 lithium equivalents for unprocessed and processed sodium alanate, respectively; theoretical value: 4 lithium equivalents) with almost 70% of Coulombic efficiency [Reference Silvestri, Farina, Meggiolaro, Panero, Padella, Brutti and Reale96]. It was suggested that the mechanical powder processing promoted the creation of a highly reactive carbon-hydride composite, which permitted the formation of a solid–electrolyte interface without Li consumption [Reference Cirrincione, Silvestri, Mallia, Stallworth, Greenbaum, Brutti, Panero and Reale149]. The composite was also able to mitigate the electrode pulverization upon cycling, which prevented the loss of electronic contact between particles upon discharge–charge cycles. The studies indicated a multistep discharge reaction that proceeded via the formation of intermediate Na₃AlH₆ and/or LiNa₂AlH₆ phases; however, the complete mechanism of the NaAlH₄ regeneration has not been fully understood. Further improvement in the performance was expected by integration of the nanoconfined sodium alanate in the electrochemical cell. In this case, the reversible conversion reaction in the Li-ion cell with the hydride-based electrode was demonstrated and high values of the initial discharge–charge capacity were reported [Reference Silvestri, Paolone, Cirrincione, Stallworth, Greenbaum, Panero, Brutti and Reale97, Reference Huen, Peru, Charalambopoulou, Steriotis, Jensen and Ravnsbæk98]. However, during the subsequent 20 cycles, the significant loss in the reactivity of nanoconfined NaAlH₄ was observed, which caused the substantial fading of the battery reversible capacity.